Field of Invention
The present invention relates to a rotor for a centrifugal flow machine and a centrifugal flow machine. The present invention is especially applicable in designing impellers for centrifugal pumps and blowers.
Background Information
In the following description of prior art and the present invention, a centrifugal pump has been used as an example of a centrifugal flow machine, and an impeller as an example of a rotor of a centrifugal flow machine. However, it must be borne in mind that the present invention may be used in connection with any centrifugal flow machine i.e. any pumping or blowing apparatus having a rotary shaft, which has a rotor coupled thereto. Thus the centrifugal flow machine includes, in addition to centrifugal pumps, also centrifugal blowers, just to name a couple of most preferred alternatives.
Nowadays centrifugal pumps or flow machines may be categorized by the type of their rotor into centrifugal flow machines having closed, semi-open or fully open impellers. When speaking in brief and somewhat simplified manner a closed impeller is an impeller, whose working vanes are at their both radially or spirally extending sides or edges covered by a shroud, a semi-open impeller has the shroud only at one radially or spirally extending side or edge of the working vanes and the open impeller does not have a shroud at all.
Traditionally centrifugal pumps have used as their shaft sealing a packing box-type sealing. However, nowadays various slide ring seals have been designed to perform the same task and occupy the same position at the rear side of the impeller. Additionally, so called dynamic seals are in use, too. In dynamic seals the sealing is taken care of by a repeller when the pump is running and a static seal when the pump is not running. However, the use of the slide ring seal has got popular and its popularity will increase in the future while the users are moving towards pumps having variable speed drives. The construction of present impellers is not able to ensure safe use of a slide ring seal, as neither the cavity or space for the sealing nor the impeller has been designed such that the sealing would, in all operating conditions of a pump, be totally surrounded by the liquid to be pumped. Additionally, the various impeller structures have to be chosen in accordance with the liquid to be pumped, and the user cannot be sure that the sealing works in a reliable manner in all possible operating conditions. The impellers comprise structures, which make the impellers hard to manufacture and decrease the efficiency ratio of the impeller. Furthermore, balancing arrangements in use at present for balancing the axial forces across the impeller waste a significant part of the efficiency ratio of the impeller.
In the following various problems concerning different impeller structures will be discussed.
EP-A2-2236836 may be mentioned as an example of a document discussing a closed impeller of a centrifugal pump. As a first problem, especially concerning small pumps, of a closed impeller, where the working vanes of the impeller are situated between two shrouds, i.e. a rear and a front shroud, the shrouds take a significant part of the cross-sectional flow area of the flow channel (between the front and rear walls of the volute).
If the impeller includes a sealing ring at the rear side of the rear shroud (the shroud farther away from the inlet of the pump), there is normally a flow connection by balancing holes through the rear shroud to the front side of the rear shroud, i.e. to the area of the working vanes. In this construction there is a flow of liquid to be pumped from the pressure side of the impeller (area at or close to the trailing edges of the working vanes) to the sealing cavity and from there via the balancing holes back to the suction side of the impeller (area at or close to the leading edges of the working vanes). The sealing space forms a chamber, which cannot be kept clean but solid matter suspended in the liquid to be pumped is received and collected in the chamber. The axial force acting on the impeller may be relatively efficiently balanced by the balancing holes.
If the impeller includes rear vanes on the rear surface (facing away from the pump inlet) of the rear shroud the impeller may be designed with or without balancing holes.
If such an impeller with rear vanes does not have balancing holes through the rear shroud, the sealing chamber is a dead-end chamber, where the liquid is not able to change and usually gas contained in the liquid is collected in the sealing chamber resulting in that the sealing is running dry and pressure is decreasing below boiling point due to the efficient work of the rear vanes. The axial force is high, when the pump is run outside its best efficiency point.
If the impeller with rear vanes has balancing holes through its rear shroud liquid is flowing to the rear side of the rear shroud via the balancing holes. This construction ensures better liquid circulation and the axial force is balanced better on a wider production range.
A semi-open impeller, sometimes also called as a half-open or a semi-closed impeller, has been discussed, as an example, in U.S. Pat. No. 5,385,442. The semi-open impeller has a flow space between the rear shroud of the impeller and a separate static rear wall, the rear wall oftentimes being a part of a casing cover of a centrifugal flow machine. In this kind of a centrifugal flow machine the rear shroud takes a significant part of the cross sectional flow area in the flow channel, too.
The semi-open impeller may have rear vanes so that the pressure acting on the rear wall is balanced close to the pressure on the front side of the shroud. However, it should be understood that only at a single operating point of the pump the axial force is fully balanced. If the semi-open impeller includes balancing holes, the same problems may be seen as with a closed impeller. And if the semi-open impeller is does not include balancing holes, the same problems may be seen as with a closed impeller, too.
If a semi-open impeller does not include rear vanes, the axial force cannot be balanced, but several bearings have to be taken in use for absorbing the axial force. If this construction has no balancing holes through the shroud, the sealing chamber is a dead-end chamber, where the liquid is not able to change and usually gas contained in the liquid is collected in the sealing chamber resulting in that the sealing is running dry. The axial force is very high. If the shroud of a semi-open impeller includes balancing holes, the sealing chamber is still a dead-end chamber, where the liquid is not able to change and usually gas contained in the liquid is collected in the sealing chamber resulting in that the sealing is running dry. The axial force is high but somewhat lower than in the construction without balancing holes.
If the semi-open impeller includes, at its rear side, a sealing ring the sealing chamber has a fluid connection to the front side of the shroud, i.e. to the suction side of the impeller, via the balancing holes. In this type of a construction, the liquid to be pumped flows from the pressure side of the impeller (at the impeller outer circumference) to the sealing chamber and therefrom via the balancing holes to the suction side of the impeller (at the inner circumference of the working vanes of the impeller). In this case, the sealing chamber is a cavity that is not able to stay clean but solids suspended in the liquid to be pumped are received and collected in the chamber. The axial force is relatively well balanced by the discussed structure.
An open impeller is an impeller where a flow channel for liquid is disposed between the impeller support disc, front wall of the volute and the static rear wall thereof. As an example of a document discussing an open impeller U.S. Pat. No. 3,964,840 may be mentioned. The impeller support disc is, in fact, a rear shroud of an impeller having a reduced diameter such that the support disc extends outwardly to a radial distance from the impeller hub and gives support to the working vanes. Normally, due to the presence of the support disc the working vanes may be made relatively thin at their root area, i.e. at their ends where they connect to the hub.
The construction of the open impeller may comprise a support disc without balancing holes. In such a construction the sealing chamber is a dead-end chamber, where the liquid is not able to change and usually gas contained in the liquid is collected in the sealing chamber resulting in that the sealing is running dry. The axial force is, however, rather well balanced.
The construction of the open impeller may, as a variant, comprise a support disc with balancing holes. In this construction liquid to be pumped flows via the balancing holes to the rear side of the support disc. The construction ensures better liquid circulation and the axial force is rather well balanced at a relatively wide production range.
U.S. Pat. No. 3,481,273 discusses another type of an open impeller where the working vanes have been attached to the hub by root portions such that there are, between the working vanes, open areas having the same diameter as the hub surface, i.e. there is no support disc for attaching the working vanes to the hub.
In brief, the various traditional rotor or impeller structures of centrifugal flow machines have a few drawbacks, which complicate the manufacture and use of the flow machines, reduce their efficiency ratio and risk the reliable and trouble-free operation of the shaft sealing.
Firstly, the closed and semi-open impeller have relatively high friction losses and limited cross sectional flow area due to the presence of the at least one shroud. Also, the efficiency ratio is affected negatively by the existence of the shroud/s.
Secondly, the existence of an axial force subjected to the impeller or rotor requires the use of larger or stronger bearings.
Thirdly, the present prior art impeller structures do not, not even the open impeller, ensure sufficient and reliable flushing of the sealing chamber.